A standard injection molding machine feeds a hot plasticized melt through a nozzle to a mold. The mold has a cavity open to the outside at a gate orifice that the nozzle is fitted to in order to inject the melt under pressure into the cavity. The nozzle is provided with a valve structure so that flow of the melt can be interrupted as the mold is opened and the previously formed workpiece is removed.
In order to insulate the nozzle, which must be kept very hot to maintain the melt molten, from the mold, which must be as cool as possible to speed curing of the plastic, the mold is formed around the gate orifice with a cup-shaped recess that is generally complementary to but somewhat larger than a top of the nozzle, so that the nozzle tip, when fitted in the recess, does not directly touch the mold. Such a system is known from DE 196 08 676. Here the melt is fed from a central melt passage via at least one a short oblique bore to the recess into whose apex opens the gate orifice. A pin in the nozzle can move between an extended position in which it projects from the nozzle tip and fits into the gate orifice and a retracted position pulled back from the gate orifice. When the pin engages in the gate orifice it has a valve function and blocks flow from the recess into the mold cavity, and when retracted it allows such flow. In this system the gap formed by the cavity between the nozzle tip and the mold has a significant insulating function, as the heat capacity of the melt is very small compared to that of the metal nozzle and mold.
U.S. Pat. No. 4,781,572 describes another such injection nozzle where the melt is fed to oblique bores via a central melt passage, the bores being provided in the nozzle tip and ending in a cup-shaped recess. Here also, the melt is fed through the recess to a gate orifice of the recess that can be closed by means of a pin. In a region adjacent the end of the pin at the mouth of gate orifice, the pin is formed around its entire circumference with an annular groove so that the pin in this region has a smaller diameter. Axial shifting can align either the small-diameter region or the bigger end region with the mouth of the gate orifice. If the bigger end region is at the mouth of the gate orifice, the gate orifice is closed, so that no melt can enter the mold cavity. If, however, the small-diameter groove region is aligned with the mouth of the gate orifice, an annular space is formed between the mouth of the gate orifice and the outside surface of the pin, and the melt can pass through the gate orifice via this space to the mold cavity. With this system, the oblique bores of the nozzle tip end close to the gate orifice of the nozzle in the recess, that is near the apex of the recess.
In practice it has been shown that during injection molding a number of molding cycles are necessary after a dye change until the molded part no longer has any of the old dye. It takes several cycles to flush all the old plastic out of the recess, wasting product and making workpieces that cannot be used.